A phase-locked loop may be represented as a combination of three basic components: a phase detector, a loop filter and a voltage-controlled oscillator (VCO), the loop filter being connected to the output signal of the phase detector and the control input of the VCO. The phase detector compares the phase of a periodic input signal or reference frequency against the phase of the signal produced by the VCO. The difference voltage signal generated by the phase detector is a measure of the phase difference between the two input signals. The difference voltage signal is filtered by the loop filter to produce a control voltage which is then applied to the VCO. Application of the control voltage to the VCO changes the frequency of an output signal produced by the VCO in a direction that reduces the phase difference between the input signal and the reference source.
One well-known type of phase detector uses interconnected digital logic gates to detect whether the phase of the VCO output signal leads or lags that of the reference signal. The voltage used to control the VCO is produced by integrating the current at a circuit node, that current being supplied by a charge pump (a paired current source and current sink) precisely controlled by the phase detector. When the phase detector detects that the phase of the output signals leads that of the reference signal, the charge pump is controlled to withdraw current from the node, reducing the control voltage and retarding the phase of the output signal. When the phase detector detects that the phase of the output signal lags that of the reference signal, the charge pump is controlled to inject current into the node, increasing the control voltage and advancing the phase of the output signal.
Historically, such phase detectors have suffered from the occurrence of a "dead zone" in their operating response, i.e., a range of phase differences in response to which the phase detector does not produce any output signal. The dead zone occurs for very small phase differences as a result of the charge pump not being activated for a sufficient time to appreciably influence the integrated output of the detector.
Before phase-lock can be attained, frequency lock must first be achieved, since signals of different frequencies by definition cannot (except instantaneously) be in phase. Frequency variation of either the reference signal or the output signal produces a phase error such that the loop is no longer phase-locked.
As the loop frequency naturally and unavoidably drifts from the reference frequency, the loop feedback mechanism cannot correct for the drift until the phase error becomes large enough to extend past the dead zone. As a result, the dead zone permits random frequency modulation as the loop frequency varies and phase error wanders from one end of the dead zone to the other, degrading the accuracy and spectral purity of the output signal. Techniques have been developed to eliminate the troublesome dead zone problem. Once such technique is described, for example, in U.S. Pat. No. 4,322,643 to Preslar, which is incorporated herein by reference.
Phase-locked loops are widely used in frequency synthesis to produce an output signal of a frequency that is a multiple of an input frequency. An ideal phase-locked loop would lock-in quickly to a particular frequency within a wide frequency range and, once locked, would not be easily untracked by noise perturbations of the reference signal. In practice, however, such performance criteria are often in conflict. For example, to realize fast lock-in, it is desirable to have a wide overall loop bandwidth. In order to prevent signal leakage from the reference oscillator and other disturbances from being input to the VCO and causing unwanted frequency component in the output signal, however, a narrow overall loop bandwidth is desirable.
The prior art provides various techniques for changing the loop bandwidth of a phase-locked loop according to operating conditions from a more acquisition-optimal bandwidth to a more tracking-optimal bandwidth. An example of such a technique is found in U.S. Pat. No. 3,909,735 to Anderson et al, incorporated herein by reference. In that patent, the output signals of a narrow band loop filter and a wide band loop filter are combined to form the control input to a voltage controlled oscillator (VCO). The output signal of the wide band loop filter is gradually attenuated by a switch control circuit during the progression of the phase-lock process. When final lock is achieved, the output signal of the wide loop filter is fully attenuated.
In such an arrangement, however, switching between different filters often causes disturbing transients which prolong the lock-in time. In addition, the timing of the switching operation may be difficult to choose and control.
What is needed is a phase-locked loop that acquires phase lock quickly but is not easily susceptible to noise perturbations and reference leakage that cause undesired spectral components in the VCO output signal. More particularly, what is needed is a phase-locked loop whose loop characteristics inherently realize the foregoing performance objectives without switching components into or out of the loops or requiring any other deliberate control operation.